1. Field of the Invention
The invention relates generally to global positioning system (GPS) positioning and more particularly to GPS positioning where a GPS-based position of a remote node is determined by a GPS node locator connected through a communication network.
2. Description of the Prior Art
The United States government maintains a global positioning system (GPS) having a constellation of earth orbiting GPS satellites. The satellites broadcast GPS signals having location-determination information that can be received and decoded by a GPS receiver for determining a GPS-based geographical location of the receiver and a GPS-based time.
The typical acquisition process for finding signal power in a GPS signal involves correlating pseudorandom (PRN) codes carried on incoming satellite signals against locally generated PRN code replicas. The code replicas are correlated at time or phase offsets with respect to a local reference time through an entire code epoch until the time or phase offset is found that provides the highest correlation. This process is known as a code search.
When signal power is found at a time or phase offset, the GPS receiver determines data bit timing from inversions of the code phase correlations between code epochs. The GPS receiver uses the data bit timing for monitoring the GPS data bits until a Z-count is decoded. A time-of-transmission for the GPS signal is read from the Z-count. The GPS time-of-transmission is used with ephemeris information that is decoded from the GPS data bits or stored locally and updated at intervals for calculating the current location-in-space of a GPS satellite. The GPS receiver uses either the location-in-space and an assumed local position with the time or phase offset, or the data bit timing with the time or phase offset for providing a pseudorange to the GPS satellite. Four pseudoranges are used with the locations-in-space of four GPS satellites for resolving the time error of the GPS reference time and the three dimensions of the geographical location of the GPS receiver.
The Z-counts for the GPS satellites are carried at six second intervals in the subframes of the GPS signal data bits. Typically, in order to ensure that random data is not mistaken for the Z-count, two subframes or slightly more than twelve seconds must be observed. Unfortunately, this requires that the GPS receiver have full power consumption for this time in order to ensure that a Z-count is detected.